Butler Fields are located near Barton Springs Rd and Lamar Blvd, located behind the Chuys on Barton Springs
On the map below, the red star marks the location of the Butler Fields. The red star also marks Toomey Road. The purple circle marks Sterzing Road.
If you are traveling on Barton Springs Rd, go north on Sterzing, and follow it until it dead ends into Toomey Road. Make a right on Toomey (which is your only option at the dead-end anyway), and the fields will be on your left. Be careful -- the first fields you encounter on your left will be some little league fields. Continue past a couple fields until you find the Butler Fields, which are the last ones on the left.
If you are traveling on Lamar Blvd, go west on Toomey, and the Butler Fields will be the first on your right.
If you want to map this using Yahoo Maps, the address of the fields is:
1614 Toomey Rd
Austin, TX